1.2.6.7- octatetraene

Figure 8-6 Approximate x-y Coordinates for the Alternant Hydrocarbons Ethene Through 1,3,5.7-Octatetraene.

The overall transformation is the conversion of the carbon-sulfur bonds bond to a carbon-carbon double bond. The original procedure involved halogenation of a sulfide, followed by oxidation to the sulfone. Recently, the preferred method has reversed the order of the steps. After the oxidation, which is normally done with a peroxy acid, halogenation is done under basic conditions by use CBr2F2 or related polyhalomethanes for the halogen transfer step.92 This method was used, for example, to synthesize 1,8-diphenyl-1,3,5,7-octatetraene. 

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

We will now consider the studies devoted to the next two linear polyenes 1,3,5-hexatriene and 1,3,5,7-octatetraene. First, we will present the results corresponding to geometries and conformational energies computed for these compounds. We will then discuss the computed frequencies and force fields. 

TABLE 2. Geometrical parameters for 1,3-butadiene (C4H6), 1,3,5-hexatriene (CcIIx), 1,3,5,7-octatetraene (CgHio) and 1,3,5,7,9-decapentaene (C10H12) from n-CAS-MCSCF calculations with 6-31G basis set17... 

A number of electrocyclic reactions have been carried out with systems of other sizes, e.g., conversion of the 1,3,5,7-octatetraene 97 to the cyclooctatriene 98.383 The stereochem-... 

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

Fig, 4. Transition structure for the electrocyclic ring closure of 1,3,5,7-octatetraene to 1,3,5-cyclooc-tatriene... 

Relatively few polyatomic ions have been observed to emit radiation, presumably because internal conversion is rapid. Fluorescence of polyatomic ions has been reported, but these ions all tend to fall into one of two groups, being either highly fluorinated molecules or highly unsaturated hydrocarbons [1—3, 84, 203, 275, 461, 512]. There is no reason to suppose that emission of radiation is a general characteristic of excited ions and the usual situation appears to be that other types of decay processes preclude emission. Fragmentation and emission of radiation appear to compete in the trans-1, 3, 5, 7-octatetraene molecular ion [461 see also ref. 207]. 

P. 5-Aza-1-metalla-1,3,5,7-octatetraenes and 2H-Pyrroles via Addition of Primary Alkenylimidates... 

Thomas IV, B. E. Evanseck, 1. D. Houk, K. N. Electrocyclic reactions of 1-substituted 1,3,5,7-octatetraenes. An ab initio molecular orbital study of torqu-oselectivity in eight-electron electrocyclizations, J. Am. Chem. Soc. 1993, 115,... 

Addition of water to dienes is catalyzed by palladium complexes. The reaction has been used for synthesizing unsaturated alcohols and ethers from aliphatic conjugated C4 and Cg olefins 248). In particular, the hydration of butadiene with water in the presence of bis(2,4-pentane-dionato)palladium and triphenylphosphine gave 2,7-octadien-l-ol, l,7-octadien-3-ol, and 1,3,5,7-octatetraene 18). The reaction was accelerated by carbon dioxide. Compounds Pd(PPh3)4 and Pd(02C0)-(PPh3)2 were also effective. 

FIGURE 2. Schematic representation of the structure of (a) all-tran.s-l,3,5-hexatiiene and (b) diR-trans-1,3,5,7-octatetraene... 

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